Contents of this Issue

Navigation

Page 56 of 92

REDEFINING QUENCHING TECHNOLOGY
HTPRO
GAS QUENCHING IS THE SAFEST, MOST ENVIRONMENTALLY FRIENDLY OF QUENCHING SYSTEMS
AND PRODUCES THE LEAST AMOUNT OF DISTORTION.
Aymeric Goldsteinas* and Jake Hamid*
Ipsen Inc., Cherry Valley, Ill.
Gas quenching has several benefits compared with other quench systems. It is
the only dry quenching that exists and,
therefore, it eliminates all environmental or safety problems connected with
liquid quenching. This article examines
why defining gas quenching in bar pressure no longer applies, why a new definition is needed, and how this definition
enables a better understanding of which
steel and what cross section can be hardened via gas quenching.
crostructure transforms into the harder
martensite phase. The cooling rate must
be fast enough to minimize formation of
softer bainite and pearlite phases, which
negatively impact mechanical properties. The key to accomplishing the transformation is uniform heat removal from
the part surface. Figure 1 shows a representative continuous cooling curve for a
ferrous alloy.
Quenching oils have a range of quenching severity depending on their physical
properties, especially viscosity. Oil, like
water, exhibits a pronounced vapor
phase upon quenching followed by a nucleate boiling phase with a very rapid
heat transfer in the temperature range of
570° to 1110°F (300° to 600°C). The three
cooling stages of an oil quench are
shown in Fig. 2.
A hardened core or hardened surface for
metals is accomplished by heating to a
sufficiently high temperature and rapid
cooling (quenching) to room temperature. Quenching uniformity is of the utmost importance, which requires
addressing quench system inadequacies
that may be detrimental to process results. Dry gas quenching meets industry
needs more efficiently than liquid
quenching.
These stages might not occur at all part
locations at the same time. During the
oil-quench nucleate boiling phase, extremely high instantaneous heat transfer coefficients can be achieved. This is
an advantage in the temperature range
where pearlitic transformation occurs,
an advantage not shared by gas quenching. However, with the breakdown of
the vapor phase at the onset of boiling,
the so-called Leidenfrost effect occurs.
State-of-the-art quenching
Heat treating ferrous metal parts involves heating to a temperature above
the upper critical temperature (Ac3) into
the austenite region of the phase diagram, which depends on alloy composition. Parts are rapidly cooled by a
quenching fluid or gas, so that the mi-
Eutectoid temperature
700
Austenite
Temperature, °C
Pearlite
4
600
1000
500
50%
800
Pearlite + Bainite
400
900
Austenite
3
2
300
Bainite
600
500
Ms
200
M50
1
M90
Martensite and austenite
100
0
700
Martensite
.01
1
10
2
3
100
Time, s
400
Bainite and
martensite
Fine pearlite
10,000
4
100,000
Fig. 1 — Continuous cooling curve for a ferrous alloy.
*Member of ASM International and ASM Heat Treating Society
54
ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013
300
1,000,000
Temperature, K
18
The result is a totally nonuniform heat
transfer rate on various surfaces of different parts, which depends on a number of variables and factors. This
uneven transitory step creates huge
temperature differentials, and is the
major factor in distortion when
quenching in these media.
Gas quenching is a pure, convective type
single-phase quench. Gas species, pressure, and velocity are the main controlling factors. Gas-quench cells are
equipped with powerful fans capable of
injecting gases (typically up to 20-bar
positive pressure) in conjunction with
heat exchangers using chilled water to
quickly remove heat from the quenching
gases. The most common quenching
media is high-pressure nitrogen gas. A
major benefit of the more uniform cooling rate of gas quenching is lower part
distortion. High-pressure gas quenching
can sometimes eliminate the need for
post-heat treatment straightening or
clamp-tempering operations, reduce
grind stock allowances and hard machining, and replace more costly
processes, such as press quenching.
Comparison of oil and gas quench rates
Gas quenching intensity can be adjusted
to match the cooling rate of liquid
quenching as shown below.
Heat transfer
coefficient
Liquid
(), W/m2K
medium
500
1000
2000
3000
Gas
Salt
10-bar N
Oil (unagitated) 20-bar He
Oil (agitated)
40-bar H
Water
100-bar H
The mean heat transfer coefficient over
total cooling in quenching a full load in
a high-volume gas flow from 1650° to
212°F (900° to 100°C) can be matched to
liquid quenching. Thus, the overall cooling rate from start to finish for quenching in 10-bar nitrogen gas (gas velocity
= 10 m/s) compares to quenching a full